Prophylactic Ketamine Attenuates Learned Fear

Abstract

Ketamine has been reported to be an efficacious antidepressant for major depressive disorder and posttraumatic stress disorder. Most recently, ketamine has also been shown to be prophylactic against stress-induced depressive-like behavior in mice. It remains unknown, however, when ketamine should be administered relative to a stressor in order to maximize its antidepressant and/or prophylactic effects. Moreover, it is unknown whether ketamine can be prophylactic against subsequent stressors. We systematically administered ketamine at different time points relative to a fear experience, in order to determine when ketamine is most effective at reducing fear expression or preventing fear reactivation. Using a contextual fear conditioning (CFC) paradigm, mice were administered a single dose of saline or ketamine (30 mg/kg) at varying time points before or after CFC. Mice administered prophylactic ketamine 1 week, but not 1 month or 1 h before CFC, exhibited reduced freezing behavior when compared with mice administered saline. In contrast, ketamine administration following CFC or during extinction did not alter subsequent fear expression. However, ketamine administered before reinstatement increased the number of rearing bouts in an open field, possibly suggesting an increase in attentiveness. These data indicate that ketamine can buffer a fear response when given a week before as prophylactic, but not when given immediately before or after a stress-inducing episode. Thus, ketamine may be most useful in the clinic if administered in a prophylactic manner 1 week before a stressor, in order to protect against heightened fear responses to aversive stimuli.

Figure 1

Prophylactic ketamine administration 1 week before CFC results in a decreased fear response. (a) Experimental design. (b) During CFC training, mice injected with saline or ketamine displayed similar levels of freezing. (c) Both groups of mice traveled comparably during each of the 3 shock presentations. (d) During the first extinction exposure, prophylactic ketamine-injected mice expressed significantly less freezing behavior when compared with saline-injected mice. Both groups of mice expressed similar levels of freezing behavior during subsequent extinction trials, reinstatement, and secondary extinction trials. (e) Ketamine-injected mice exhibited significantly less fear during extinction than saline-injected mice (n=5 mice per group). Error bars represent±SEM. **p<0.01, and ***p<0.001. CFC, contextual fear conditioning; E, extinction; K, ketamine; R, reinstatement; Sal, saline.

Figure 2

Prophylactic ketamine administration 1 month or 24 h before CFC does not result in a decreased fear response or facilitate extinction, but increases immobility if administered immediately before CFC. (a and b) Experimental design. (c) During CFC, mice injected with saline or ketamine 1 month before CFC displayed similar levels of freezing. (d) Both groups of mice traveled comparably during each of the 3 shock presentations. (e) Both groups of mice exhibited similar levels of freezing behavior during CFC training and during extinction (n=5 mice per group). (f) Mice injected with saline or with ketamine 24 h before CFC expressed equal levels of freezing behavior during CFC training. (g) Saline- and ketamine-injected mice traveled equally during the shock presentation. (h) Both groups of mice displayed similar levels of freezing behavior during all but one extinction trial (n=5 mice per group). (i) During CFC, mice injected with ketamine 1 h prior have increased immobility before the shock presentation when compared with saline-injected mice. (j) Ketamine-injected mice travel less during the shock presentation than saline-injected mice. (k) However, both groups express equal levels of freezing behavior during almost all extinction trials. (n=10 mice per group). Error bars represent±SEM. *p<0.05, and **p<0.01. CFC, contextual fear conditioning; E, extinction; K, ketamine; R, reinstatement; Sal, saline.

Figure 3

Ketamine administration does not facilitate extinction when administered following CFC. (a) Experimental design. (b) Mice injected with saline or with ketamine 1 h after CFC express equal levels of freezing behavior during all extinction trials (n=5 mice per group). (c) Experimental design. (d) Mice injected with saline or with ketamine 1 week after CFC, but 1 week before extinction, expressed equal levels of freezing behavior during all extinction trials (n=10 mice per group). (e) Experimental design. (f) Mice injected with saline or with ketamine 1 h before extinction expressed equal levels of freezing behavior during all extinction trials (n=5 mice per group). Error bars represent±SEM. CFC, contextual fear conditioning; E, extinction; K, ketamine; R, reinstatement; Sal, saline.

Figure 4

Ketamine administration following extinction does not buffer against subsequent fear-inducing stimuli but increases attentiveness. (a) Experimental design. (b) Mice injected with saline or with ketamine 24 h after the last extinction trial expressed equal levels of freezing behavior during reinstatement and during secondary extinction trials. (c) Both groups of mice traveled comparably during the reinstatement shock presentation. (d) Both groups of mice exhibited equal levels of immobility in the FST on day 1 and on day 2. (e) Both groups of mice exhibited equal levels of movement in the OF. (f) Ketamine-injected mice displayed an increased number of rearing bouts when compared with saline-injected mice (n=5 mice per group). (g) Experimental design. (h) Saline and ketamine-injected mice displayed comparable levels of fear behavior during reinstatement and during secondary extinction trials. (i) Both groups of mice traveled comparably during each of the 3 reinstatement shock presentations. (j) Both groups of mice exhibit equal levels of immobility in the FST on day 1 and on day 2. (k and l) Both groups of mice exhibited equal levels of movement in the OF and had a comparable number of rearing bouts (n=9–10 mice per group). Error bars represent±SEM. *p<0.05. CFC, contextual fear conditioning; E, extinction; FST, forced swim test; K, ketamine; OF, open field; R, reinstatement; Sal, saline.

Figure 5

Ketamine administration following reinstatement decreases fear following three-shock CFC, but not one-shock CFC. (a) Experimental design. (b) Mice injected with saline and ketamine 1 h after one-shock reinstatement express comparable levels of fear behavior during secondary extinction trials (n=5 mice per group). (c) Experimental design. (d) Mice injected with ketamine 1 h after three-shock reinstatement express decreased fear when compared with mice injected with saline (n=9–10 mice per group). (e) Mice injected with saline or ketamine 24 h after three-shock reinstatement express comparable levels of freezing behavior during secondary extinction trials (n=5 mice per group). Error bars represent±SEM. *p<0.05, and **p<0.01. CFC, contextual fear conditioning; E, extinction; K, ketamine; R, reinstatement; Sal, saline.